Inverter circuits for converting DC voltages into AC driver voltages for driving essentially capacitive loads, such as electroluminescent lamp devices, for example, normally utilize single-ended output driver circuits which are relatively inefficient, i.e., they provide a relatively low output to input power ratio and produce relatively low power sufficient to produce luminescence only over a relatively small area of luminosity.
Capacitive loads have a low power-factor, that is, the voltage and current are out of phase by a phase angle approaching 90 degrees. Conventional internally excited inverters, used to drive capacitive loads in a continuous manner, suffer from the fact that the transformer drive circuit therein usually attempts to drive a low power factor, e.g., capacitive, load with drive circuitry that is optimized for resistive loads, which have a power factor essentially equal to one and in which the voltage and current are in phase. This mismatch between the drive circuit and load results from the fact that the transformer can only present feedback at a phase angle of 0.degree. or 180.degree. with respect to the primary excitation. If the feedback transformer winding is then used to drive the transformer power drive circuitry, high peak currents occur in the transformer so as to cause a saturation thereof. The high peak currents provide no useful output because the transformer, being saturated at this time, can couple no additional power to the output.
Attempts at controlling such transformer saturation have conventionally been made by simply limiting the drive to a single sided drive transistor. Because the single sided driver doesn't control the transformer saturation over an entire cycle, however, the fundamental frequency, drive waveform is distorted, thereby producing harmonic output signals having frequencies higher than the desired drive fundamental frequency, and the overall efficiency is reduced. The distortion of the drive waveform causes a even further penalty when used for driving electroluminescent lamps. Because of the physical construction of electroluminescent lamps, frequencies higher than the desired drive frequency result in unwanted losses due to heating and premature aging or deterioration of the lamps.
Inverters used in power supply applications, or inverters that use single-ended switching devices, such as diodes or transistors, at their outputs effectively disconnect the output during one-half of each cycle. The resulting waveform is non-sinusoidal and contains high frequency harmonics that are not only not usable but are detrimental to an electroluminescent lamp, as mentioned above. Inverters for this type of application attempt to fully saturate the transformer so that, by doing so, the linear losses associated with the switching devices are reduced. Because an electroluminescent lamp cannot use the harmonic components that are generated in a fully saturated switching device, this type of inverter does not provide optimally effective operation for use as an electroluminescent lamp driver circuit. DC to AC converters that work into resistive loads can function at relative high efficiencies because resistive loads can utilize the harmonic content as usable power. When such converters are used with capacitive type loads, which operate in response to a single fundamental frequency, such as in light emitting devices, particularly of the electroluminescent type, the efficiency thereof is reduced.
Inverter driver circuits having double-ended outputs, i.e., ones using a dual driver circuitry, have been proposed for use with electroluminescent devices. One such circuit is made by Hero Electronics, Ltd. of Bedfordshire, England, in their Model D10542 inverter device. Such driver circuitry uses a relatively standard dual transistor drive circuit supplied from a DC voltage source which circuit requires a specially designed transformer that does not use standard laminations and bobbins. While the transformer secondary output winding provides an AC output driver voltage, the circuitry has a relatively low efficiency and produces a driver voltage for providing luminescence only over a relatively small area, e.g., less than 10 square inches.
The principal drawback to such a circuit is that no means is provided to control the saturation of the output transformer. Therefore, when the inverter attempts to provide power to an electroluminescent lamp, the drive signal is starting when the transformer is already saturated. Further, such circuitry, as in conventional single-ended circuits used for such purpose, is subject to high current surges which undesirably saturate the transformer windings, the circuit also being subject to producing undesirable high frequency oscillations superimposed on the output driver signal. Moreover, when the load of the electroluminescent device is removed, a sufficiently high current surge may occur as to adversely affect, or damage, the circuit.
Accordingly, it is desired to provide inverter driver circuitry for electroluminescent devices which uses relatively few components to achieve cost reductions, has improved efficiency and sufficient power to provide luminescence for areas as high as 80-100 square inches, or more, for example, and uses a conventional output transformer configuration having standard laminations and bobbins. Such circuitry should be designed so as to provide better efficiency and to avoid the generation of undesired high frequency oscillations.